The present invention relates to optical devices and, more particularly, to a tunable microlaser and other monolithically integrated Fabry-Perot cavity optical devices, as well as an optical interconnect for processors.
The advent of microlasers (sometimes referred to in the art as vertical-cavity surface emitting lasers) has been met with much excitement. A microlaser is many orders of magnitude smaller than a conventional diode laser. This miniaturization of the lasing phenomena is important in its own right, as well as providing other advantages. Moreover, microlasers and other Fabry-Perot cavity devices are fabricated by relatively well known techniques, the same techniques used to fabricate integrated circuitry. The article entitled "Microlasers" appearing on page 86 et seq. of the November 1991 issue of Scientific American provides a good overview of microlasers and such article is incorporated herein by reference.
One difficulty with microlasers is that they are relatively inflexible. That is, they typically emit only one optical frequency of optical radiation. (The term "optical radiation", as used herein, means electromagnetic radiation in the visible wavelength spectrum and in other adjacent wavelength spectrums--typically radiation having a wavelength in the range of between 10 and 15,000 nm.) In this connection, a microlaser is a type of semiconductor laser. Many semiconductors exhibit optical gain when adequate electrical or optical stimulation is provided. This gain region can be quite broad in energy or wavelength, and displays a maximum that depends on the strength of the external stimulation and the nature of the semiconductor. A conventional waveguide semiconductor laser consists of a relatively long Fabry-Perot cavity with mirrors spaced on the order of 200 .mu.m apart. For such a long cavity, the Fabry-Perot resonances are closely spaced in wavelength, and, in the simplest case, the lasing wavelength of the device occurs at the resonance wavelength with the maximum gain. Such a device can be tuned to different wavelengths by adjusting the temperature of the semiconductor, since the maximum gain shifts in wavelength as the temperature changes. This method has a very limited tuning range, and the tuning time is on the order of seconds.
An alternative method of tuning conventional semiconductor lasers is to fabricate distributed mirrors at the ends of the waveguide. These mirrors are only reflective at a narrow range of wavelengths that depends on the spacings in the distributed mirror. Lasing occurs at the Fabry-Perot mode where the reflectivity is highest in the mirrors. The effective spacing (effective optical distance) between the mirrors and therefore the lasing wavelength can be changed in such a structure by modulating the refractive index in the distributed mirror regions. This can be accomplished by non-linear optical effects, such as carrier induced index modulation. Generally, such methods cause heating and parasitic frequency changes. Since non-linear optical effects tend to be very weak, and they limit the tuning range of such devices to less than about 10 nm in wavelength.
In contrast to the above, the Fabry-Perot cavity of a microlaser is very short, on the order of a few wavelengths, and thus the mirrors of such a laser are made to have much higher reflectivities to compensate for the lower total gain. Microlasers are fabricated on a substrate by either being grown epitaxially or by deposition, and the mirrors consist of quarter-wave periods of semiconductor for the bottom mirror, and some combination of quarter-wave semiconductor or dielectric layers as the top reflector. Since the cavity is very short, the Fabry-Perot modes are spaced far apart, and to get any lasing at all, at least one Fabry-Perot mode must be in the gain region of the semiconductor in the cavity.